Ocean microplastic levels change with the seasons
A new method can spot ocean microplastics on a global scale and track them over time.
The new work provides a day-to-day timeline showing where microplastics enter water, how they move, and where they tend to accumulate.
An estimated 8 million tonnes of plastic waste enters the ocean each year, and most of it is transformed by the sun and waves into microplastics, tiny specks that can overlap currents in the hundreds or thousands. kilometers from their point of entry.
Debris can harm marine life and marine ecosystems, and it is extremely difficult to track and clean up.
The new approach is based on the Cyclone Global Navigation Satellite System, or CYGNSS, and can zoom in or zoom in on small areas for a high resolution image of microplastic releases from a single location.
The technique is a major improvement over current tracking methods, which rely primarily on the uneven ratios of plankton trawlers cleaning up microplastics with their catch.
“We are still at the beginning of the research process, but I hope this can be part of a fundamental shift in the way we monitor and manage microplastic pollution,” said Chris Ruf, professor of climate science. and space at the University of Michigan, principal investigator. by CYGNSS, and lead author of a new paper on research into IEEE Transactions on Geosciences and Remote Sensing.
Great Pacific Garbage Patch and the Yangtze River
The team found that global microplastic concentrations tend to vary seasonally, peaking in the North Atlantic and Pacific during the summer months of the northern hemisphere. June and July, for example, are the peak months for the Great Pacific Garbage Patch, a convergence zone in the North Pacific where microplastic accumulates in massive amounts.
Concentrations in the southern hemisphere peak during the summer months of January and February. Concentrations tend to be lower in the winter, possibly due to a combination of stronger currents that break up the plumes of microplastics and increased vertical mixing that pushes them further below the water’s surface, according to the researchers. researchers.
The data also showed several brief spikes in microplastics concentration at the mouth of the Yangtze River, long believed to be the main source.
“It’s one thing to suspect a source of microplastic pollution, but quite another to see it happen,” says Ruf. “The data on microplastics that were available in the past was so scarce, just brief snapshots that are not reproducible.”
The researchers produced visualizations that show the concentrations of microplastics around the world. Often, areas of accumulation are due to dominant local water currents and convergence zones, with the Pacific area being the most extreme example.
“What makes the plumes of major river mouths remarkable is that they are a source in the ocean, as opposed to places where microplastics tend to accumulate,” says Ruf.
The information could help organizations that clean up microplastics deploy ships and other resources more efficiently, Rus says. Researchers are already in talks with a Dutch cleaning organization, Ocean Cleanup, to work together to validate the team’s initial findings. Single point release data may also be of use to the United Nations agency, UNESCO, which sponsored a task force to find new ways to track microplastic releases to the world’s waters.
The roughness of the ocean
Undergraduate student Ruf and Madeline Evans developed the tracking method that uses existing data from CYGNSS, an eight microsatellite system launched in 2016 to monitor weather conditions near the core of large storm systems and strengthen forecasts on their seriousness. Ruf leads the CYGNSS mission.
Key to the process is the roughness of the ocean surface, which CYGNSS is already measuring using radar. The measurements were primarily used to calculate the wind speed near the eyes of hurricanes, but Ruf wondered if they might have other uses as well.
“We had taken these radar measurements of surface roughness and used them to measure wind speed, and we knew that the presence of substances in the water impairs its reactivity to the environment,” says Ruf. “So I came up with the idea of doing everything backwards, using changes in reactivity to predict the presence of things in the water. “
Using independent measurements of wind speed from NOAA, the team looked for places where the ocean seemed less choppy than it should be given the wind speed. They then compared these areas with actual observations of plankton trawlers and ocean current models that predict microplastic migration. They found a strong correlation between smoother areas and those with more microplastic.
Ruf’s team believe that microplastics may not be directly responsible for changes in ocean roughness, but rather could be surfactants, a family of oily or soapy compounds that lower the temperature. surface tension on the surface of a liquid. Surfactants tend to accompany microplastics in the ocean, both because they are often released with microplastics and because they similarly travel and accumulate once in the water.
Source: University of Michigan